Signature of heavy Majorana neutrinos at a linear collider: Enhanced charged Higgs pair production

Abstract

A charged Higgs pair can be produced at an $ee$ collider through a $t$-channel exchange of a heavy neutrino ($N$) via ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{H}^{+}{H}^{\ensuremath{-}}$ and, if $N$ is a Majorana particle, also via the lepton number violating (LNV) like-sign reaction ${e}^{\ifmmode\pm\else\textpm\fi{}}{e}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\rightarrow}{H}^{\ifmmode\pm\else\textpm\fi{}}{H}^{\ifmmode\pm\else\textpm\fi{}}$. Assuming no a priori relation between the effective $eN{H}^{+}$ coupling ($\ensuremath{\xi}$) and light neutrino masses, we show that this interaction vertex can give a striking enhancement to these charged Higgs pair production processes. In particular, the LNV ${H}^{\ensuremath{-}}{H}^{\ensuremath{-}}$ signal can probe $N$ at the International Linear Collider (ILC) in the mass range $100\text{ }\text{ }\mathrm{GeV}\ensuremath{\lesssim}{m}_{N}\ensuremath{\lesssim}{10}^{4}\text{ }\text{ }\mathrm{TeV}$ and with the effective mixing angle $\ensuremath{\xi}$ in the range ${10}^{\ensuremath{-}4}\ensuremath{\lesssim}{\ensuremath{\xi}}^{2}\ensuremath{\lesssim}{10}^{\ensuremath{-}8}$---well within its perturbative unitarity bound and the $\ensuremath{\beta}{\ensuremath{\beta}}_{0\ensuremath{\nu}}$ limit. The lepton number conserving ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{H}^{+}{H}^{\ensuremath{-}}$ mode can be sensitive to, e.g., an $\mathcal{O}(10)\text{ }\text{ }\mathrm{TeV}$ heavy Majorana neutrino at a 500 GeV ILC, if ${\ensuremath{\xi}}^{2}\ensuremath{\gtrsim}0.001$.